The present invention aims to propose a method for symmetrizing asymmetric defects which may occur within a traction chain comprising a synchronous or asynchronous motor controlled by a voltage inverter.
Inverters, and more particularly voltage inverters, are intended to provide an AC voltage from a DC voltage.
A particularly important field of application is the variable-speed control of synchronous or asynchronous machines.
In this case, it is necessary for the load, which may be represented by each phase of a synchronous or asynchronous motor, to be provided with a three-phase voltage system which is as close as possible to a frequency-varying and amplitude-varying balanced sinusoidal three-phase system. The voltage inverter is a device which makes it possible to achieve this goal, and is made up of static switches using power components such as thyristors. Until very recently, among these power components, use was made essentially of semiconductors such as thyristors, GTOs, etc.
For some time, new types of static switches have been appearing which may be defined by the term xe2x80x9cIGBTxe2x80x9d (Insulated Gate Bipolar Transistor). These devices are controllable components, in the sense that one can at any moment prescribe the current required in the switch by adjusting the voltage on its control gate, whereas for the old generation of switches represented for example by GTOs or the like, it is possible only to decide the moment at which they turn on and the moment at which they turn off.
The present invention relates more particularly to inverters using the new generation of power components such as IGBTs or even bipolar transistors.
When a semiconductor is short-circuited (failure of a semiconductor) in a two-level inverter, the corresponding phase of the motor is connected permanently either to the top point of the inverter, or to the bottom point thereof, provided that the semiconductor situated on the same branch is off, otherwise the entire branch is short-circuited, this amounting to short-circuiting the input and the outputs of the inverter. At this moment, the action of blocking the remainder of the inverter by turning off all the unimpaired semiconductors will place the motor in an electrical configuration which may (depending on the instant of the defect) generate a transient excess torque at the motor shaft, causing the snapping of the mechanical transmission.
The solution to this problem is to cause a symmetric three-phase short-circuit at the terminals of the motor. To do this, several possibilities exist.
In the particular case of the short-circuiting of an asynchronous motor, it is known practice in particular from the document SU-1350783 to simultaneously short-circuit all the phases of a motor, so as to avoid an asymmetric defect, that is to say a defect linking a phase to the top point and to the bottom point of the power supply to the inverter. It is necessary to produce a symmetric three-phase short-circuit of the asynchronous motor so as not to load the mechanical transmission excessively by the excess torque generated by asymmetric short-circuits.
The simplest solution consists in turning on all the semiconductors of the inverter simultaneously, thus causing the short-circuiting of the input capacitor of the inverter. However, in this case, the semiconductors must be dimensioned so as to be able to withstand the short-circuit current of the inlet capacitor in addition to the short-circuit current of the motor. This is possible with GTO-based inverters, since the current in these transistors is limited by limiting inductances (di/dt). Unfortunately, in the case of IGBT-based inverters, the use of low-inductance busbars between the inlet capacitor of the inverter and the IGBTs results in the current not being limited by inductances as is the case for GTOs. Moreover, the IGBT itself limits its collector current by desaturating. These two reasons mean that this strategy is not easily transposable to IGBT-based inverters.
On the other hand, protection devices for IGBT-based two-level inverters powering a variable-speed motor exist in industry. Document GB-A-2309597 summarizes the state of the art in this area.
The devices in question use circuits for detecting desaturation of the IGBTs to detect a defect inside or at the output of the inverter and to disable the inverter.
The drawback of these devices is that they may generate an asymmetric defect on the asynchronous motor powered by the inverter protected by these same devices.
The present invention aims to propose a method for protecting a synchronous or asynchronous traction chain, equipped with a voltage inverter, itself made up of static switches consisting of semiconductors, and which solves the problem of the capability of the said semiconductors to withstand current.
The present invention aims more particularly to be able to limit the torque of a synchronous or asynchronous motor powered by a two-level or multi-level voltage inverter generated during a defect located inside or outside the inverter powering the motor.
The present invention therefore aims to propose a strategy for the overall protection of the traction chain which makes it possible to symmetrize an asymmetric short-circuit at the output of a voltage inverter without damaging the latter.
The present invention relates to a method for symmetrizing an asymmetric defect which may occur within a traction chain comprising a synchronous or asynchronous motor controlled by a two-level or multi-level voltage inverter made up of static switches using semiconductors such as IGBTs or bipolar transistors, according to which one detects whether the defect connects or might connect a motor phase to the top point or to the bottom point of the power supply of the inverter, and in that one short-circuits only that half of the inverter exhibiting the defect on the basis of the result of the detection.
The present invention relates more precisely to a method of protection in which all the phases of the voltage inverter are connected permanently to the top point or to the bottom point of the inverter, thereby causing a symmetric three-phase short-circuit at the output of this inverter without causing any short-circuit on its input after detecting and locating a defect. This implies that all the phases of the motor are linked to the top point of the power supply of the inverter, if the defect is such that it links at least one motor phase to the top point of the power supply of the inverter, and that all the phases of the motor are linked to the bottom point of the power supply of the inverter, if the defect is such that it links at least one motor phase to the bottom point of the power supply of the inverter, this being achieved without causing any short-circuit on the input of the inverter.
According to the form of execution using a two-level inverter, the defect in a two-level inverter is located by using the principle of the detection of desaturation of the IGBT semiconductor.
According to the form of execution using an inverter with three or more levels, the defect in a three-level inverter is located by using the principles of the detection of desaturation and of the detection of active peak-limiting of the IGBT semiconductor.